The invention is directed to a process of recovering .epsilon.-caprolactam from reaction mixtures containing .epsilon.-caprolactam and sulfuric acid. Typically, these reaction mixtures may result from two different methods of producing .epsilon.-caprolactam, which is used to produce nylon-6. One of the synthetic routes for producing .epsilon.-caprolactam involves subjecting cyclohexanone oxime to a Beckmann rearrangement reaction by treating that oxime with sulfuric acid, oleum or sulfur trioxide. Another route involves reacting cyclohexane carboxylic acid with a nitrosating agent in the presence of sulfuric acid.
Because of the importance of .epsilon.-caprolactam, the art has developed various methods for recovering .epsilon.-caprolactam from sulfuric acid containing reaction mixtures. In all these methods, disadvantages inhere. For instance, U.S. Pat. No. 2,993,889 is directed to removing sulfuric acid from .epsilon.-caprolactam containing mixtures by neutralizing the sulfuric acid with ammonia water. The disadvantage of this process is that large unwieldy amounts of ammonium sulfate are produced as by-product which cannot serve a useful purpose. U.S. Pat. No. 3,336,298 attempts to overcome that drawback of U.S. Pat. No. 2,993,889 by converting the sulfuric acid of the .epsilon.-caprolactum containing reaction mixture into ammonium hydrogen sulfate rather than ammonium sulfate; concentrated solutions of ammonium hydrogen sulfate can be used in the dissolution of rock phosphate. In order to convert the sulfuric acid into ammonium hydrogen sulfate, the later patent teaches reacting the sulfuric acid with ammonium sulfate. This process obviates formation of ammonium sulfate by-product, but the efficiency of recovery of .epsilon.-caprolactam is then dependent on the production of an other product.
U.S. Pat. No. 3,852,272 obviates the result of formation of useless by-products by partly neutralizing the sulfuric acid in said mixtures with ammonia water, extracting the lactam from the partly neutralized mixture; and burning the ammonium hydrogen sulfate produced in said partial neutralization to produce sulfur dioxide which can be converted into sulfuric acid. The disadvantage of this method resides in the loss of the ammonia since during burning it is reduced to nitrogen and water. U.S. Pat. No. 3,879,380 obviates the formation of unuseful by-products and the combustion of ammonia, by partially neutralizing the sulfuric acid in .epsilon.-caprolactam containing mixtures to form a melt containing ammonium hydrogen sulfate and treating said melt with a suitable metal oxide, e.g. zinc oxide to recover ammonia and sulfur trioxide. This method or recovering ammonia and sulfur trioxide is extremely expensive.